I had written the first (defconstant ...) line.
It didn't work at all. I thoroughly checked and rechecked all
other aspects of my program. I thought I was thoroughly checking
the results of decode also. It turns out though that unless
you're very keen, you don't really think much of the extra quote
mark in the output of the (decode-flags ...) routine.

Essentially though, rather than (decode-flags ...)
returning a list that contains :foo for example, it
returns a list which contains the symbol QUOTE
and a sub-list which contains :foo. Oops.

Almost two years in, I am still getting used to when I have to quote things
and when I don't. This is especially difficult when it comes
to type-specifiers. *shrug*